Known heating manners in a continuously treating line for a steel band are a direct flame heating system and an indirectly heating system using radiant tubes. The former is more excellent in heating efficiency in comparison with the latter, and since it may burn out a cold rolling oil, a cleaning facility may be saved, it has been used widely in a hot-dip zinc plating line, a continuously annealing line for electromagnetic steel plates, and others.
However, in the conventional direct flame heating system, oxidation of the steel band is remarkable, and there arises a big problem causing roll peakup thereby. For such conventional ones, so-called direct flame heating system without causing oxidation has been proposed for continuously heat treating facilities in Japanese Patent Publication No. 44,133/83 and Japanese Patent Publication No. 29,651/84. These systems lower air ratios (less than 1.4.fwdarw.0.6) in combustion control zones in response to heightening of strip temperatures (Max. 900.degree.), and carry out the heating while controlling oxidation of the steel band.
These systems are called as non-oxidation, but actually they are slight oxidation systems. Since oxidizing gases such as C.sub.2 or H.sub.2 O are much contained in gases generated in combustion, thickness of oxidized film after heating comes up to from less than 50 .ANG. to 500-1000 .ANG.. Therefore, when such a system is applied to the continuously annealing facility, it is inevitable that hydrogen in the atmosphere is made high in concentration about 20%) in a soaking zone following a heating zone, so as to reduce the oxidized film, otherwith the direct flame heating furnace is, at its exit, furnished with a treating zone serving as a forcibly reducing zone, so as to reduce the oxidized film with hydrogen of high concentration (more than about 50%).
Continuously annealing facilities recently developed have become large scaled aiming at decreasing production costs. In these facilities if the above mentioned direct flame heating zone were built in 1 PASS (a steel band runs once within the heating zone), a furnace would be very tall, and problems would arise that the steel band flutters as running or controlling of the furnace is difficult. Therefore, the heating zone could not be but built in 2 PASS or more. However, in such direct flame heating zone of plural PASSes, the roll within the heating zone is caused with roll pickup due to the oxidized film and the surface appearance of the steel band is remarkably spoiled. For avoiding the roll pickup, complicated instruments should be required, for example, an isolation chamber filled with a protecting atmosphere for housing the roll within the furnace, as disclosed in Japanese Patent Application Laid Open No. 53-54100 (1978). Also in the roll protecting system, it is very difficult to exactly provide a sealing between the roll isolation chamber and the heating furnace at high temperature. The protecting atmospheric gas must be much supplied to fully charge the protecting atmosphere in the isolation chamber. As a heating treatment in the continuously hot-dip zinc plating line, so-called Senzimir system is known. In the continuously annealing cycle, this system heats the steel by directly flaming it up to about 400.degree. to 450.degree. C. and subsequently indirectly heats it up to about 750.degree. C. in a forcibly reducing atmosphere. But this system has difficulties as under.
(1) indirect heating time is long, and heating efficiency is bad,
(2) forcibly reducing atmosphere, i.e. (H.sub.2 +N.sub.2) having high H.sub.2 concentration required, and a problem arises about safety, and
(3) when the soaking is performed, the length of the furnace is made large.
In view of Senzimir system, another system has been developed and now used widely which carries out a slight oxidation heating in so-called NOF furnace and subsequently does reduction by indirect heating in a following weak reduction furnace. But this system also has difficulties as under.
(1) indirect heating time is long,
(2) in the weak oxidation-weak reduction process, a surface activity of the steel band is not fully obtained which is required to plating adhesion, and
(3) the length of the furnace is made large.
In addition, the conventional continuously annealing line has further problems as follows.
1st problem: In the continuously heat treating furnace installed with the direct flame heating furnace, the steel band is heated up to 500.degree. to 800.degree. C. without causing oxidation or with reduction, and subjected to the treatments of heating, soaking and rapid cooling, and it is cooled down less than about 150.degree. where oxidation in the atmosphere is no problem, and left in the air. The furnace is maintained therein in positive pressure of about +5 to 15 mmWC so as to prevent the invasion of the atmosphere and is charged with the atmospheric gas (3 to 20% H.sub.2 and the rest being N.sub.2) for reduction and prevention of oxidation.
For a sealing mechanism which leads out the heat-treated steel band in the air, a seal roll system and a water envelop system are taken into consideration. But these systems have difficulties as under.